Deoxyhalogeno Sugars

On the regioselectivity of the Hanessian-Hullar reaction in 4,6-O-benzylidene protected galactopyranosides

The N-bromosuccinimide mediated fragmentation of methyl 4,6-O-benzylidene-beta-D-galactopyranoside results in the formation of methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside and methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside, as opposed to the methyl 6-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside originally reported. The kinetic methyl 4-O-benzoyl-6-bromo-6-deoxy-beta-D-galactopyranoside rearranges to the thermodynamic methyl 4-O-benzoyl-3-bromo-3-deoxy-beta-D-gulopyranoside under the reaction conditions, likely via a 3,6-anhydro galactopyranoside. The NBS-mediated cleavage of 4,6-O-benzylidene acetals in the galactopyranoside series is therefore shown to conform to the regiochemistry observed in the corresponding gluco- and mannopyranoside series with preferential cleavage of the C6-O6 bond by an ionic mechanism.

Stereocontrolled synthesis of the D- and L-glycero-beta-D-manno-heptopyranosides and their 6-deoxy analogues. Synthesis of methyl alpha-l-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl- (1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L- rhamno-pyranoside, a tetrasaccharide subunit of the lipopolysaccharide from Plesimonas shigelloides

The synthesis of d- and l-glycero-alpha-manno-thioheptopyranosides, protected with 4,6-O-alkylidene-type acetals is described. In glycosylations carried out with preactivation with the 1-benzenesulfinylpiperidine/trifluoromethanesulfonic anhydride couple, both the D- and L-glycero series exhibit excellent beta-selectivity with a range of glycosyl acceptors. In contrast, a 4,7-O-alkylidene acetal was found not to afford beta-selectivity. With a 4,6-O-[1-cyano-2-(2-iodophenyl)ethylidene] acetal protected thioglycoside, excellent beta-selectivity was obtained in glycosylation reactions, and subsequent treatment with tributyltin hydride and azoisobutyronitrile brought about clean fragmentation to the 6-deoxy-glycero-beta-D-manno-heptopyranosides. This chemistry was applied to the stereocontrolled synthesis of methyl alpha-L-rhamno-pyranosyl-(1-->3)-D-glycero-beta-D-manno-heptopyranosyl-(1-->3)-6-deoxy-glycero-beta-D-manno-heptopyranosyl-(1-->4)-alpha-L-rhamno-pyranoside, a component of the lipopolysaccharide from Plesimonas shigelloides.

Influence of monosaccharide derivatives on liver cell glycosaminoglycan synthesis: 3-deoxy-D-xylo-hexose (3-deoxy-D-galactose) and methyl (methyl 4-chloro-4-deoxy-beta-D-galactopyranosid) uronate

An improved, convenient synthesis of 3-deoxy-D-xylo-hexose (3-deoxy-D-galactose) has been developed, and the chemical synthesis of a novel monosaccharide derivative, methyl (methyl 4-chloro-4-deoxy-beta-D-galactopyranosid)uronate (compound 10), is described. Using primary hepatocytes in culture, each was used to explore its effect on glycosaminoglycan (GAG) synthesis. In the absence of analogues hepatocytes synthesize primarily (92-95%) heparan sulphate. At 1 mM, 3-deoxy-D-galactose had little observable effect on either liver cell GAG or protein synthesis. At 10 mM and 20 mM, 3-deoxy-D-galactose reduced [3H]glucosamine and 35SO4 incorporation into hepatocyte cellular GAGs to, respectively, 75% and 60% of the control cells. This inhibition of GAG synthesis occurred without any effect on hepatocyte protein synthesis, indicating that 3-deoxy-D-galactose's effect on GAG synthesis is not mediated through an inhibition of proteoglycan core protein synthesis. Furthermore, GAGs in the presence of 20 mM of the analogue were significantly reduced in size, 17 kDa vs. 66 kDa in untreated cells. These results reflect either impaired cellular GAG chain elongation, and/or altered GAG chain degradation. Compound 10 exhibited a concentration-dependent inhibition of both hepatocyte cellular GAG and protein synthesis. At concentrations of 5, 10 and 20 mM, compound 10 inhibited GAG and protein synthesis by 20, 65 and 90%, respectively. Exogenous uridine was able to restore partially the inhibition of protein synthesis, but was unable to reverse the effect of compound 10 on GAG synthesis. These results show that part of the effect of compound 10 on GAG synthesis is not mediated by an inhibition of proteoglycan core protein synthesis. GAGs in the presence of compound 10 are half as large as those in the absence of this compound (33 and 66 kDa, respectively). These results again may reflect either impaired cellular GAG chain elongation and/or altered GAG chain degradation. Potential metabolic routes for each analogue's effect are presented.

Halogenation reactions of derivatives of D-glucose and sucrose

Treatment of methyl 4,6-O-benzylidene-alpha-D-glucopyranoside (1) with triphenylphophine-carbon tetrachloride-pyridine (reagent A) gave methyl 4,6-O-benzylidene-2-chloro-2-deoxy-alpha-D-mannopyranoside (2). When reagent A was used in excess, a further elimination reaction occurred to give methyl 4,6-O-benzylidene-2-chloro- (6, 60%) and -3-chloro-2,3-dideoxy-alpha-D-erythro-hex-2-enopyranoside (7, 16%). Treatment of 1 with triphenylphosphine-carbon tetrabromide-pyridine (reagent B) caused little or no elimination, and 47% of methyl 4,6-O-benzylidene-2-bromo-2-deoxy-alpha-D-mannopyranoside (14) was obtained. On treatment with reagent A, methyl alpha-D-glucopyranoside (16) gave exclusively methyl 2,4,6-trichloro- 2,3,4,6-tetradeoxy-alpha-D-erythro-hex-2-enopyranoside (17), and methyl 4,6-O-benzylidene-beta-D-glucopyranoside (19) gave methyl 4,6-O-benzylidene-3-chloro-3-deoxy-beta-D-allopyranoside (20, 70%). However, with reagent B, 19 gave methyl 4,6-O-benzylidene-3-bromo-3-deoxy-beta-D-glucopyranoside (23, 66%), probably by way of double inversion of configuration at C-3. Likewise, with reagent A, methyl beta-D-glucopyranoside (25) gave methyl 2,4,6-trichloro- (26) and3,4,6-trichloro-2,3,4,-6-tetradeoxy- beta-D-threo-hex-2-enopyranoside (27), and 4,6-O-isopropylidenesucrose (28) gave mainly 3-chloro-3-deoxy-4,6-O-isopropylidene-alpha-D-allopyranosyl 1,4,6-trichloro-1,4,6-trideoxy-beta-D-lyxo-hexulofuranoside (29) together with 3-chloro-3-deoxy-4,6-O-isopropylidene-alpha-D-allopyranosyl 1,4,6-trichloro-1,4,6-trideoxy-beta-D-fructofuranoside (30). The assignment of structure to 29 is tentative.